Rheumatoid arthritis (RA) is a frequent and chronic inflammatory disease of the synovial joints. The citrulline posttranslational modification is forme by the conversion of peptidylarginine residues into the citrulline amino acid by peptidylarginine deiminase (PAD) family members. PAD enzymes have been implicated in RA pathology because many RA autoantibodies are directed against citrullinated proteins. The development of autoantibodies to citrullinated epitopes in RA suggests that aberrant PAD activity contributes disease. PAD2 expression is closely linked with inflammation in RA synovial tissue. Interestingly, PAD2 KO mice exhibit ameliorated disease in a serum-transfer model of inflammatory arthritis. Mast cells contribute to disease pathogenesis in experimental models of inflammatory arthritis, and we have identified mast cells as a major source of the PAD2 enzyme. We find that activation of the P2X7 receptor by the inflammatory danger signal ATP induces robust protein citrullination in a PAD2-dependent manner. The overall HYPOTHESIS to be evaluated is that activation of mast cell-derived PAD2 by an inflammatory danger signal contributes to inflammatory arthritis. Specific aims: The specific aims of this proposal are to delineate the P2X7R/PAD2 pathway in mast cells and to determine the contribution of mast cell derived PAD2 to inflammatory arthritis. Significance: These studies will provide insight the mechanism of PAD2 activation in RA and will support our long-term goal to develop methods for the prevention and treatment of RA based on understanding the molecular mechanisms underlying RA pathogenesis.